Free wheels are known, which can rotate in a single direction independently with respect to a rotary member, for example a shaft, on which they are mounted coaxially, while, when the direction of rotation is inverted, they are clamped on said member, so as to rotate solidly therewith. The limitation of free wheels is that their rotation, independent with respect to the member on which they are mounted, can occur, as we said, only in one of the two directions, since the rotation in the opposite direction automatically implies that the wheel is clamped on said member.
In some applications it becomes necessary to use transmission devices with wheels able to rotate independently from the relative rotating mounting member in both directions, and be clamped, selectively and on command, on said member in order to make the rotation thereof uniform.
In order to achieve this condition, complex transmission devices have been proposed which provide to use external clamping elements such as joints, jaws or suchlike, and do not in any case allow, during the clamping step, sufficiently rapid response times.
WO-A-82/02869 discloses a coupling device for the coupling up or uncoupling of a drive roller for a coupling conveyor. The coupling device comprises first and second coupling members. The second coupling member comprises a plurality of coupling hook elements, pivotally mounted about their pivot shafts in a holder. Each of the coupling hook is constructed in the form of a swinging lever with a hooking portion and a control portion. The coupling device also includes a control arm which comprises a control portion. The control arm is adapted to be displaced along a guide, parallel to the direction of the tangent to a control ring, between a lower position in which the coupling device is in the coupling position, and an activation position in which the control arm causes a radial displacement of the control ring in a position which is eccentric in relation to the pivot shaft. This causes the uncoupling between the first and the second coupling member, because the second coupling member ceases to rotate while the first coupling member can continue its rotation.
A major shortcoming of this device is the out-of-center (eccentric) and asymmetrical positioning of the control ring with respect to the pivot shaft in order to obtain the uncoupling between the first and the second members.
AT-B-142.282 discloses a coupling/uncoupling device able to be activated only above a pre-determined rotational speed. The two bodies are always meshed each other and they disengaged when the rotational speed reaches a pre-set value. Moreover, when the two bodies are engaged each other, they can rotate in one sense only.
Applicant has devised and embodied the present invention to overcome these shortcomings of the state of the art and to obtain other advantages.